Hello,
I have the same exact problem with the Internet Box Standard.
I am quite annoyed because I spent hours looking for the problem and, then, I've seen that the router updated by itself and this problem occurred.
I think the correct term for it is "hairpinning", you can look on Wikipedia.
We can just wait for a bugfix or a guide for the setup.
A possible workaround is to force your DNS to use the IPV6 address, in that case you will see that you can access the website even from the LAN.
I have also written in the Italian section, I will report if someone answers.